System and method for providing variable delay FIR equalizer for serial baseband communications

ABSTRACT

The present invention relates in general to a method, apparatus, and article of manufacture for providing high-speed digital communications through a communications channel. In one aspect, the present invention employs variable delay FIR equalizer in the transmitter module to increase the system performance in channel communications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/313,478, entitled “Variable Delay FIR Equalizer for SerialBaseband Communications”, filed Aug. 20, 2001, and U.S. ProvisionalApplication Ser. No. 60/313,214, entitled “Transceiver Apparatus andMethod”, filed Aug. 17, 2001. The contents of these provisionalapplications are incorporated, in their entirety, by reference herein.

This application is related to U.S. Provisional Patent Application Ser.No. 60/313,454, entitled “Transceiver System for High Speed DigitalSignaling”, filed Aug. 20, 2001; U.S. Provisional Patent ApplicationSer. No. 60/313,455, entitled “Automatic Slicer Level Adaptation”, filedAug. 20, 2001; U.S. Provisional Patent Application Ser. No. 60/313,456,entitled “Variable Rate Sub-Channel Using Block Code RDS”, filed Aug.20, 2001; U.S. Provisional Patent Application Ser. No. 60/313,477,entitled “Crosstalk Management for High-Speed Signaling Links”, filedAug. 20, 2001; and U.S. Provisional Patent Application Ser. No.60/313,477, entitled “Method and Apparatus for Encoding and DecodingDigital Communications Data”, filed Aug. 20, 2001. These applicationsare hereby incorporated herein by reference.

This application is also related to non-provisional patent applicationsthat claim priority to one or more of the above-referenced provisionalpatent applications. These non-provisional patent applications areentitled “System and Method for High Speed Digital Signaling”, filedAug. 16, 2002 (application Ser. No. 10/222,122); “System and Method forProviding Slicer Level Adaptation”, filed Aug. 16, 2002 (applicationSer. No. 10/222,073); “System and Method for Embedding a Sub-Channel ina Block Coded Data Stream”, filed Aug. 16, 2002 (application Ser. No.10/222,071); “System and Method for Providing Crosstalk Management forHigh-Speed Signaling Links”, filed Aug. 16, 2002 (application Ser. No.10/222,072); and “Method and Apparatus for Encoding and Decoding DigitalCommunications Data”, filed Aug. 16, 2002 (application Ser. No.10/222,254). The aforementioned non-provisional applications are herebyincorporated by reference, in their entirety, herein.

BACKGROUND OF THE INVENTION

This application relates in general to a method, apparatus, and articleof manufacture for providing high speed digital communications through acommunications channel, and more particularly to a method, apparatus,and article of manufacture for providing a variable delay finite impulseresponse equalizer for baseband communications.

Digital communications systems are continuously increasing the transferrate at which data is transmitted between devices through acommunications channel, for example, a backplane. To meet that increase,conventional systems have employed certain techniques of increasing therate of the transmitted signal or increasing the number of bits persymbol while maintaining the transmission rate. The first approach isproblematic in that most distortions that plague communications systemsincrease with increasing frequency and thus the fidelity of the receivedsignal is degraded. The second approach is also problematic because thevoltage margin is reduced when including more information pertransmitted symbol. The result is that either approach may require animprovement in received signal quality to maintain a given quality ofservice.

A common approach to address this situation is to utilize an equalizer(adaptive or otherwise) to compensate for the increased distortionand/or the increased sensitivity to distortion. However, including anequalizer adds cost, complexity and power consumption to a receiver ortransceiver. Thus, there is a strong desire to develop an equalizerstructure that can provide the required compensation with a minimum ofcomplexity.

Typically, a conventional symbol spaced equalizer implemented as adiscrete-time system can correct for a channel pulse response durationor length of N*Tsym seconds, where N is the number of taps of theequalizer and Tsym is the symbol period. A fractionally spaced equalizerimplemented as a discrete-time system can correct for a channel pulseresponse duration or length of N*Ttap seconds, where N is the number oftaps of the equalizer and Ttap is the tap period which is a fraction ofa symbol period.

However, in certain situations, a subset of the taps in the equalizermay be zero. One such situation may occur in systems in which daughtercards are plugged into a motherboard. Thus, it may be desirable toeliminate the circuitry for taps that would converge to zero. However,after design and manufacture, conventional symbol spaced equalizers havelimited flexibility when implemented within a particular environment.

Thus, there is a need for an improved variable equalizer in order toenhance the system performance of, for example, high-speed digitalcommunications through a communications channel, for example abackplane. There is a need for an equalizer with improved flexibilityand spanning a large time range without requiring the circuitry andcomplexity of the intermediate taps that may not be used in a givensituation.

SUMMARY OF THE INVENTION

The present invention relates in general to a method, apparatus, andarticle of manufacture for providing high-speed digital communicationsthrough a communications channel, for example a backplane.

In one aspect, the present invention is an equalizer structure thatconsists of one or more taps that are movable. This equalizer structuremay be implemented in the transmitter and/or the receiver to enhance theperformance of the communications system.

The equalizer structure of the present invention may also permit a spanof a large time range without the burden of unnecessary circuitry andcomplexity of intermediate taps that may not be employed (for a given orall situations). In short, increased performance in channelcommunications may be achieved using a variable delay finite impulseresponse (“FIR”) equalizer pursuant to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the course of the detailed description to follow, reference will bemade to the attached drawings. These drawings show different aspects ofthe present invention and, where appropriate, reference numeralsillustrating like structures, components and/or elements in differentfigures are labeled similarly. It is understood that variouscombinations of the structures, components and/or elements other thanthose specifically illustrated are contemplated and within the scope ofthe present invention.

FIG. 1 illustrates an exemplary communications channel between twodigital processing devices according to one embodiment of the presentinvention;

FIG. 2 illustrates an exemplary communications channel and portion of atransmitter/receiver module pair operating in accordance with oneembodiment of the present invention;

FIG. 3 illustrates a back channel communications path, including a backchannel data frame, in accordance with one embodiment of the presentinvention;

FIG. 4 illustrates a conventional equalizer structure;

FIG. 5 illustrates a waveform of a pulse transmitted through acommunications channel and the “received” pulse response;

FIG. 6 illustrates a block diagram of a communications channel and aportion of a transmitter/receiver module pair, including an equalizerstructure, according to one aspect of the present invention; and

FIG. 7 illustrates a communications channel, in conjunction withtransmitter/receiver module pairs, in accordance with one embodiment ofthe present invention.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanied drawings, which form apart hereof, and which is shown by way of illustration, specificexemplary embodiments of which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims.

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The term “connected” means a direct connectionbetween the items connected, without any intermediate devices. The term“coupled” means either a direct connection between the items connected,or an indirect connection through one or more passive or activeintermediary devices. The term “circuit” means either a single componentor a multiplicity of components, either active and/or passive, that arecoupled together to provide or perform a desired function. The term“signal” means at least one current, voltage, or data signal. The term“module” means a circuit (whether integrated or otherwise), a group ofsuch circuits, a processor(s), a processor(s) implementing software, ora combination of a circuit (whether integrated or otherwise), a group ofsuch circuits, a processor(s) and/or a processor(s) implementingsoftware.

FIG. 1 illustrates an exemplary communications channel between twodigital processing devices according to one embodiment of the presentinvention. Digital processing devices 101 and 102, such as personalcomputers, communicate with each other by transmitting digital signalsthrough communications channel 100. In one embodiment, a digitalrepresentation of the data to be transmitted is encoded and transformedinto an electronic signal capable of passing through communicationschannel 100. The electronic signal is transmitted by transmitter 110 toreceiver 111. The received electronic signal, which may be distortedwith respect to the electronic signal transmitted into or onto thechannel by transmitter 110, is processed and decoded by receiver 111 toreconstruct a digital representation of the transmitted information.

The communications channel 100 may be, for example, constructed usingone or more cables, wires, traces or the like, or may be part of abackplane, or may be a wireless communications medium through which thesignal passes from transmitter 110 to receiver 111. One skilled in theart will recognize that any such communications media, when used inconjunction with a corresponding transmitter/receiver pair appropriatefor a particular medium, may be used to construct a communicationschannel in accordance with the present invention. For example, otherchannels that may be implemented in the present invention includeelectronic, optical or wireless. Indeed, all types of channels ofcommunication (i.e., communication channels), whether now known orlater-developed, are intended to be within the scope of the presentinvention.

FIG. 2 illustrates an exemplary communications channel according to oneembodiment of the present invention. The communications channel 100enables or facilitates transmission of information between associatedpairs of transmitters and receivers, for example, transmitter 110 andreceiver 111. In one embodiment, transmitter 110 and receiver 111 employa multilevel pulse amplitude modulated (PAM-n) communications technique.For example, transmitter 110 and receiver 111 may employ a PAM-4signaling technique to send two bits of data through channel 100. Thatis, the transmitter/receiver pair is used to send two bits of data foreach symbol transmitted through the channel 100. A ten bit word isloaded into parallel-to-serial register 211 with the output of theregister being a two bit pair that is transmitted through channel 100.Once received, the sequence of two bit codes are loaded intoserial-to-parallel register 212 to reconstruct or decode the ten bitword.

In this particular embodiment, the transmitter and receiver pair employa PAM-4 communications technique to send the two bits of data throughchannel 100. Each transmitter and receiver pair operates in the samemanner; that is, each pair sends data through the channel in a serialfashion that packages two bits into one symbol. Five successive symbolsare associated with each eight bit data byte. The additional overheadassociated with this form of encoding is used to ensure adequate symbolcrossings, necessary for timing recovery, and to provide DC balance onthe line.

In addition, that overhead may also be used to transmit controlinformation for controlling or modifying certain modules or circuitry ofthe communications system, for example an adaptive or adjustableequalizer in a transmitter. In addition, that overhead may also be usedto allow the embedding of control information for controlling ormodifying certain modules of the communications system, for example anadaptive equalizer in a transmitter. Thus, control information is thatdata which is used to control, modify, adjust, enhance, optimize, and/orinitialize or re-initialize the operation, performance or function ofvarious components or modules of the transceivers, receivers andtransmitters of the system that are coupled via communications channel100. The adaptive equalizer in the transmitter is one such component ormodule.

Although the present invention is described in the context of PAM-4signaling techniques, the present invention may utilize other modulationformats that encode fewer or more bits per symbol codes based on otherthan byte wide user may be readily adapted or employed. Moreover, othercommunications mechanisms that use different encoding tables, other thanfour levels, or use other modulation mechanisms may also be used. Forexample, PAM-5, PAM-8, PAM-16, CAP, wavelet modulation and otherencoding rates such as 16B9Q or 9B5Q (among others) could be utilized.In this regard, the techniques described herein are in fact applicableto any and all modulation schemes, including but not limited to, PAM-4encoding described herein.

FIG. 3 illustrates one embodiment of the present invention. Thecommunications system typically possesses a number of unidirectionaltransmitter and receiver pairs (transmitter 110 a and receiver 111 a;and transmitter 110 b and receiver 111 b). Transmitter 110 a andreceiver 111 b may be incorporated into transceiver 305 (in the form ofan integrated circuit). Similarly, transmitter 110 b and receiver 111 aare incorporated into transceiver 306. From a system level perspective,there is a plurality of such transmitter/receiver pairs in simultaneousoperation, for example, eight or nine transmitter/receiver pairs,communicating across communications channel 100.

In operation, the transmitter and receiver pairs simultaneously transmitdata across channel 100. As mentioned above, the additional overheadassociated with the particular encoding techniques may be used fortransmitting control information in a back channel communications path.In one embodiment, the back channel forms a part of the user datachannel. In this way, back channel data may be transmittedasynchronously at the same time user data is transmitted withoutreducing or significantly impacting the amount of channel communicationscapacity dedicated to user data.

The back channel data may provide information to an adaptive oradjustable equalizer to enhance or optimize the operation of theequalizer for a given environment. In this regard, the adaptiveequalizers reside in each of the transmitters 110 a and 110 b. Thecontrol information for the equalizer in transmitter 110 a (i.e., backchannel 2) is embedded in the user data channel 1. The controlinformation for the equalizer in transmitter 110 b (i.e., back channel1) is embedded in the user data channel 2.

The back channel data is typically sent in a back channel data frame ordata packet 320. In one embodiment, data frame 320 includes frame header321, a set of data bits 322, a set of control bits 323, and data frametrailer 324. The frame header 321 is used to mark the beginning of adata frame to allow the transmitter and receiver to remain synchronizedas to the proper beginning of the data frame. The set of data bits 322contains the data to be transmitted across the back channel and isdistinguished from the set of control bits 323 used to control theoperation of the back channel as necessary. Finally, data frame trailer324 is used to mark the end of a data frame to further allow thetransmitter and receiver to remain synchronized as to the proper end ofthe data frame.

It should be noted that other message formats and features, such aserror correction or detection, may be implemented in the back channelframe. Indeed, any and all formats, whether now known or laterdeveloped, are intended to be within the scope of the present invention.Moreover, it is possible to construct an arbitrarily complex frame forthe back channel information and have the frame carried by the subchannel described herein.

With reference to FIG. 4, a conventional symbol-spaced equalizerimplemented as a discrete-time system is capable of correcting for achannel pulse response duration or length of 5*Tsym seconds. Afractionally spaced equalizer is achieved by replacing the delay cellsby other than unit delays. Many combinations of unit and non-unit delaysare possible. The exemplary received pulse response shown in FIG. 5 maybe much longer than a few symbol periods due to remote reflections, likethose arising from impedance mismatches distributed along a channel.

It should be noted that the illustrated received pulse response may also“dispersed” as a result of non-flat frequency response and group delayon the communications channel. As such, the transmitted pulse may“spread-out” thereby creating intersymbol interference.

Moreover, the remote reflections on the line may create an image of theprimary received pulse delayed by an arbitrary number of symbol periods.For the case of a backplane application, this delay may extend up to 18symbol periods (M=18) or more. Thus, a conventional equalizer wouldrequire at least 18 taps for symbol spaced, or more than 18 taps forfractional spacing, to “see” and thus correct for reflected pulse(s).

Indeed, for the situation illustrated in FIG. 5, the optimal values forthe coefficients of the intermediary taps may be zero or very close tozero. Thus, it may be desirable to eliminate the circuitry for taps thatconverge to zero.

The present invention employs an adaptive equalizer that includes one ormore taps with a controllable displacement in time to minimize the needfor, and thus the circuitry associated with, the intervening taps. Inone aspect of the present invention, an equalizer includes at least oneroving tap in combination with a fixed equalizer. In another aspect, thefixed structure equalizer coefficients (H0, H2), the roving tap locationand/or coefficient M and Hr may be being generated or determined usinginformation calculated or processed at the receiver module. In thisregard, the adaptive equalizer may reside in the transmitter or thereceiver. Indeed, in certain circumstances, it may be advantageous toimplement an equalizer (variable delay FIR and/or fixed) in both thetransmitter and the receiver.

In one embodiment, the transmitter module implements an equalizerstructure that is realized as the summation of pseudo-differentialcurrents through termination impedances at the output stage of thetransmitter. Each pseudo-differential current coming from a thermometerencoded DAC—one thermometer DAC per filter tap. This embodiment may beparticularly well suited to PAM-4 encoding techniques, in that each DACmay be realized as follows: use six equal strength current sources(three on the “plus” side and three on the “minus” side).

In operation, to represent the PAM-4 values of “−3, −1, +1, +3”, thethermometer coded DAC turns on three of the six current sources. Forexample, a “+3” is generated by turning on the three “plus” currentsources and turning on none of the three “minus” current sources (i.e.,keeping the three “minus” current sources off). Further, “+1” isgenerated by turning on two of the “plus” current sources and one of the“minus” current sources. In contrast, a “−1” is generated by turning onone of the “plus” current sources and two of the “minus” currentsources. Finally, a “−3” is made by turning on none of the “plus”current sources (i.e., keeping the three “plus” current sources off) andturning on the three “minus” current sources. The variable amplitude isrealized by digitally controlling the strength of the current sourcescomprising the thermometer encoded DAC(s).

The number of delays for the data that is presented to each of thethermometer encoded DAC(s) determines its position in the filter. Eachfilter tap value is realized by modulating the strength of the currentsources for the particular thermometer encoded DAC representing thatfilter tap. Thus, by employing this structure, there are no parallelstages for zero taps connected to the output. As such, the outputcapacitance of this structure is reduced. For high-speed applications,extra capacitance may be prohibitive because it may reduce thetransmission bandwidth. Additionally, reducing power dissipation and diearea are also concerns addressed by a system design in accordance withthe present invention.

In another embodiment, the equalizer structure may be implementeddigitally. In this embodiment, the equalizer structure performs themultiplication and summation digitally (using, for example, a processor)and presents the resulting digital words (filtered outputs) to onemulti-bit DAC. In this embodiment a reduction in computation is a directbenefit of the roving tap.

Thus, in accordance with one aspect of the present invention, thetransmitter and/or receiver includes (1) an equalizer having at leastone roving tap and a fixed tap structure; and/or (2) the fixed structureequalizer coefficients (H0, H2), the roving tap location and/orcoefficient M and Hr being generated or determined using informationcalculated or processed at or by the receiver module. In thosecircumstances where the coefficient information is determined usinginformation calculated or processed at or by the receiver module, thecoefficient information may be provided to the transmitter (and,ultimately to the adaptive equalizer located in the transmitter) via theback channel to control, enhance, modify or optimize the operation ofthe system.

FIG. 6 illustrates a block diagram representation of the physicalstructure of an equalizer in accordance with the present invention. Theequalizer of FIG. 6 is implemented in the transmitter and includes a FIRfilter having four taps. Three of the four taps have a variablecoefficient (H0, H2 and Hr) value and one of the taps has a variabledelay element (Z^(M)). The tap with the variable delay element is theroving tap.

With continued reference to FIG. 6, the roving tap (Hr) is summed at thetransmitter output with the “fixed” equalizer output (H0, 1, H2). In onepreferred embodiment, the values of the equalizer coefficients androving tap position (H0, H2, Hr, and M) are adjusted to reduce thecorrelation of the error, the difference between the received pulseand/or delayed versions of the received signal decided data D(n), i.e.,e(n)=x(n)−D(n), with delayed versions of the received data x(n−m). Thismay be referred to as the cost function of the error. Each tap receivesits own cost function calculation.

The tap adjustments or tap values may be initially determined during anauto-negotiation (AN), which is implemented at power-up or on demand. Inaddition, the tap adjustments may be performed intermittently,periodically or continuously during normal data transmission. In anotherembodiment, the tap adjustments may occur intermittently, periodicallyor continuously during normal data transmission and/or during an ANprocedure. In the preferred embodiment (as shown in FIG. 6), thereceiver module contains an automatic slicer level (ASL) module.

It should be noted that receiver module and the ASL module are describedin detail in U.S. Provisional Patent Application Ser. No. 60/313,455entitled “Automatic Slicer Level Adaption”, filed Aug. 20, 2001, andnon-provisional patent application entitled “System and Method forProviding Slicer Level Adaption”, filed Aug. 16, 2002 (application Ser.No. 10/222,073). As mentioned above, these applications are incorporatedby reference herein in their entirety.

It should be further noted that the Auto-Negotiation protocol isdescribed in detail in U.S. Provisional Patent Application Ser. No.60/313,454, entitled “Transceiver System for High Speed DigitalSignaling”, filed Aug. 20, 2001 and non-provisional patent applicationentitled “System and Method for High Speed Digital Signaling”, filedAug. 16, 2002 (application Ser. No. 10/222,122). As mentioned above,these applications are incorporated herein by reference in theirentirety.

In the preferred embodiment, the cost function calculation and tapadjustments are determined or performed digitally in a multi-stepprocedure. In the first step, the cost function for H2 is determined.The cost function for H2 is computed by multiplying the present error byx(n−1), (m=1), taking the sign of the product and accumulating the signof the product. The number of cycles of accumulation is highlyapplication specific. Typically, the number of cycles is greater than1000. In the preferred embodiment, the number of cycles is 1023(N=1023). If the accumulated correlation is positive, the value of H2 isdecreased by one. If the correlation is negative, the value of H2 isincreased by one. This adjustment is stored temporarily in the receiverwhile the adjustment for H0 is performed.

The procedure for H0 is the same as for H2, except that the correlationis with the leading data; x(n+1), m=−1, is multiplied by the presenterror. A negative value of m is realized by adding in extra delay to theerror. At the end of the procedure, if the accumulated correlation ispositive, the value of H0 is decreased by one; if the correlation isnegative, the value of H0 is increased by one. Once the adjustments forH2 and H0 have been calculated, they are sent to the transmitter and thetransmitter equalizer values of H0 and H2 are adjusted (see FIG. 7 whereRx ‘B’ sends updated coefficients to Rx ‘A’ for use in Tx ‘A’ via a lowdata rate).

It should be noted that in one embodiment, the above steps are repeated127 times to converge H0 and H2 before moving to the next step. Forother applications, the number of times these steps are repeated may behigher or lower.

The next step in the AN process is to determine a value for M, the delayfor the rover. To do this, the above procedure for computing theaccumulated correlation for H2 is repeated for each of the possibledelay values that M can take on (m is in the range of 2 to M_(max)) andthese values are stored in registers. The value for M is chosen to bethe position that produces the largest absolute value accumulatedcorrelation. The value of M is sent to the transmitter and thetransmitter equalizer value of M is set (see FIG. 7 where Rx ‘B’ sendsupdated coefficients to Rx ‘A’ for use in Tx ‘A’ via a low data rate).This process “fixes” the delay for the roving tap coefficient in time,and creates a particular structure for the equalizer.

In a preferred embodiment, the delay M is realized using a digitalpipeline stage of length M_(max) with the output of each stage formingone of the M_(max) inputs of a mulitplexer. The value of M, the desireddelay, is used to address the multiplexer. The output of the multiplexeris also gated so that no output is available prior to setting theaddress of the multiplexer to M.

The final step in the receiver AN process is to compute the final valuesfor the equalizer coefficients (H0, H2, Hr). The same procedure that wasused to compute adjustments to H2 and H0 and send them back to thetransmitter is utilized with respect to equalizer coefficients (H0, H2,Hr)—with the addition that adjustments to Hr are also computed (usingthe value x(n−M)) and sent back the transmitter as well.

Due to possible correlation between H0, H2 and Hr, the process ofdetermining the coefficient adjustments is repeated multiple times. Inthe preferred embodiment, the procedure is repeated 127 times (eachcoefficient has its adjustment computed 127 times).

After the structure of the equalizer is determined and implemented,normal data communications over channel 100 may commence.

It should be noted that there are many techniques for determining thecoefficients of equalizer. All techniques for determining thecoefficients, whether now known or later developed, are intended to bewithin the scope of the present invention.

In another embodiment of the present invention, the equalizer mayinclude a plurality of roving taps. Under those circumstances where theequalizer includes a plurality of roving taps, one technique fordetermining the positions of the roving taps (using the nomenclature Nrto indicate the number of roving taps) is to use the procedure forcomputing the position of a single roving as stated above. The positionsof the Nr largest absolute value accumulated correlations determine theNr rover tap positions.

Another technique for determining the positions of the roving taps is tocompute the rover positions in a sequential fashion, one at a time.After each rover position is determined, the equalizer is re-converged,i.e., all of the fixed taps and the rover taps with determined locationsare converged as per the method of the preferred embodiment. Theprocedure is repeated Nr times until all of the rover locations aredetermined. It should be noted that in this technique, after a positionof a given roving tap is determined, it may be advantageous to notassign later determined roving taps to the same location as earlierdetermined rovers.

In those situations, applications or environments having knownreflections, a subset of the rover locations may be positioned orlocated at fixed or substantially fixed offsets. In this regard, aroving tap which is located at the substantially fixed offset may moveor rove, if at all, a small distance from a given locations in smallincrements. The roving taps located at the substantially fixed offsetswould address the impact of the known reflections. Permitting suchroving taps to move a small distance about a fixed or known offsetallows the system to make minor adjustments for slight variationsbetween the environments or to accommodate manufacturing variations ofthe components (implemented in the channel having known reflections) orthe channel.

In another embodiment, the rover position (positions) may be partly orentirely manually pre-programmed and the rover tap weight (weights) maybe partly or entirely manually pre-programmed. Thereafter, any or allrover locations may be changed during normal data operation tocompensate for changes in environmental conditions.

In yet another embodiment, the rover position (positions) and/or therover tap weight (weights) are fixed to predetermined values stored in,for example a ROM or EEPROM. In certain situations, however such valuesmay also be fine tuned to enhance the system performance. In thisregard, after (or during) the performance of an initialization orreinitialization process, the system may implement fine adjustments tothe rover position (positions) and/or the rover tap weight (weights).The fine adjustments to the position and weight may be accomplishedusing any of the techniques described above. Indeed, all techniques fordetermining these levels, whether now known or later developed, areintended to be within the scope of the present invention.

It should be noted that additional embodiments arise by changing theform of the correlation in any of the embodiments to, including but notlimited to, sign-sign, sign-value, value-sign, and value-value (as shownhere) to generate the accumulated correlation. This would cover thestorage of the residual error in both analog and digitalimplementations. Additional embodiments arise by substituting D(n−m) forx(n−m) in any of the above embodiments.

Further, the equalizer may be located in the receiver in addition to, orinstead of in the transmitter. In this regard, an equalizer isincorporated directly into the receiver. The “receiver” equalizer ofthis embodiment may be the same as or similar to the transmitterequalizer described above. Alternatively, the “receiver” equalizer maybe implemented as a mixed signal equalizer with an analog delay line.Moreover, the equalizer of this embodiment may be a single ADC coupledto a processor or other digital logic circuits, similar to thosedescribed above.

In another embodiment, the roving taps may be grouped as a set whosecontrollable displacement in time is arranged or determined as a group.The group may consist of two, three or more taps. In this embodiment,the group of taps move together to adjust or modify the transmit signalso that the received signal is of better quality for the giventransmission environment.

In one example, the group consists of three roving taps. Here, a firstroving tap of the group may be characterized as a coarse adjustment ofthe transmit signal, a second roving tap of the group may be the mediumadjustment, and a third roving tap of the group may be the fineadjustment. In this example, the first roving tap is positioned, placedor selected to make major modifications to the transmit signal. Thesecond roving tap is positioned, placed or selected to make furthermodifications to the transmit signal or to compensate for overshoot orundershoot caused by or as a result of the first roving tap. Finally,the third roving tap may make fine adjustments to compensate for slightovershoot or undershoot caused by the previous placement, positioning orselection of the first and/or second roving taps of the group of rovingtaps.

In another example, the group may consist of two roving taps. In thisembodiment, the first roving tap of the group may perform coarseadjustment of the transmit signal, and the second roving tap of thegroup may perform fine adjustment.

Other configurations or arrangements of the roving taps are possible. Inthis regard, the number of roving taps of the group may be selectedbased on additional cost, complexity and/or power consumption to atransmitter or transceiver.

In another embodiment, any number of roving taps of the group may beenabled or disabled as necessary or desired. In this regard, as thesignal is equalized, it may be determined that the number of roving tapsin the group need not be equal to the total number of taps in the group.Thus, if a roving tap is not necessary or used in the equalizationprocess because, for example, elevated power consumption or limitedpower availability, it may be “turned off” or disabled. For example,where the group consists of three roving taps, and it is determined thatthe roving tap that performs medium adjustment of the transmit signal isnot necessary, the transceiver may employ only those taps that performcoarse and fine adjustment of the transmit signal.

Thereafter, it may be determined that medium adjustment of the transmitsignal is advantageous or necessary, the roving tap of the group thatperforms medium adjust may be “turned on” or enabled. This may result orbe due to a detected change in the received signal as the environment ofthe transmission channel changes. The roving tap coefficients may becalculated using any of the techniques described above. Moreover, thereare many other techniques for determining the coefficients of equalizer.Thus, all techniques for determining the coefficients, whether now knownor later developed, are intended to be within the scope of the presentinvention.

FIG. 1 illustrates an exemplary operating environment in which thepresent invention may be implemented. The operating environment is onlyone example of a suitable operating environment and is not intended tosuggest any limitation as to the scope of use or functionality of theinvention. Other well known computing systems, environments, and/orconfigurations that may be suitable for use with the invention include,but are not limited to, personal computers, server computers, hand-heldor laptop devices, multiprocessor systems, microprocessor-based systems,programmable consumer electronics, network PCs, minicomputers, mainframecomputers, distributed computing environments and data communicationsystems that include any of the above systems or devices, and the like.

The invention may also be described in the general context ofcomputer-executable instructions, such as program modules, executed byone or more computers or other devices. Generally, program modulesinclude routines, programs, objects, components, data structures, etc.that perform particular tasks or implement particular abstract datatypes. Typically, the functionality of the program modules may becombined or distributed.

A processing device coupled to a communications channel 100 (viatransceivers) typically includes at least some form of computer readablemedia. Computer readable media can be any available media that can beaccessed by these devices. By way of example, and not limitation,computer readable media may comprise computer storage media andcommunication media. Computer storage media includes volatile andnonvolatile, removable and non-removable media implemented in any methodor technology for storage of information such as computer readableinstructions, data structures, program modules or other data. Computerstorage media includes, but is not limited to, RAM, ROM, EEPROM, flashmemory or other memory technology, BC-ROM, digital versatile disks (DVD)or other optical storage, magnetic cassettes, magnetic tape, magneticdisk storage or other magnetic storage devices, or any other mediumwhich can be used to store the desired information and which can beaccessed by processing devices.

Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism and includesany information delivery media. The term “modulated data signal” may becharacterized as a signal that has one or more of its characteristicsset or changed in such a manner as to encode information in the signal.By way of example, and not limitation, communication media includeswired media such as a wired network or direct-wired connection, andwireless media such as acoustic, RF, infrared and other wireless media.Combinations of any of the above should also be included within thescope of computer readable media.

Additionally, the embodiments described herein may be implemented (inpart) as logical operations performed by programmable processingdevices. The logical operations of these various embodiments of thepresent invention are implemented (1) as a sequence of computerimplemented steps or program modules running on a computing systemand/or (2) as interconnected machine modules or hardware logic withinthe computing system. Accordingly, the logical operations making up theembodiments of the invention described herein can be variously referredto as operations, steps, or modules.

While the above embodiments of the present invention describe a variabledelay FIR equalizer for serial baseband communications, one skilled inthe art will recognize that the use of a particular version of an FIRfilter and delay are merely example embodiments of the presentinvention. In an alternative embodiment, the incorporation of a feedbackelement into the equalizer structure, making it into an Infinite ImpulseResponse (IIR) structure, may be employed in situations where thechannel to be equalized demands a long tail on the equalizer in additionto having remote reflection sources. In this embodiment, it would bemore likely to make some of or all of the numerator (feed forward) tapsof the equalizer roving. However, a sparse structure for the denominator(feedback) taps could also be utilized either solely or in conjunctionwith roving numerator taps. It is to be understood that otherembodiments may be utilized and operational changes may be made withoutdeparting from the scope of the invention.

As such, the foregoing description of the exemplary embodiments of theinvention has been presented for the purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not with this detaileddescription, but rather by the claims appended hereto. The presentinvention is presently embodied as a method, apparatus, and article ofmanufacture for providing a variable delay FIR equalizer for serialbaseband communications.

1. A system for transmission of data through a communication channel between a first transmitter and a first receiver, comprising: a fixed tap module, located in the first transmitter, including at least one fixed tap location; a roving tap module, located in the first transmitter, including at least one movable tap location; a set of adjustable delay coefficients, including at least one adjustable delay coefficient for at least one movable tap location; a set of adjustable filter coefficients, including at least one adjustable filter coefficient for at least one movable tap location and at least one adjustable filter coefficient for at least one fixed tap location; and wherein at least one adjustable filter coefficient, of the set of adjustable filter coefficients, is determined by the first transmitter in response to an adjustment received from the first receiver through a back channel within an overhead associated with an encoding technique.
 2. The apparatus according to claim 1 wherein the communications channel is a backplane.
 3. A system for transmission of data through a communication channel between a first transmitter and a first receiver, comprising: a fixed tap module, located in the first transmitter, including at least one fixed tap location; a roving tap module, located in the first transmitter, including at least one roving tap that has a variable delay element; a set of adjustable coefficients, including at least one adjustable filter coefficient for at least one movable tap location, at least one adjustable filter coefficient for at least one fixed tap location and at least one adjustable delay coefficient for at least one movable tap location; wherein the adjustable delay coefficient is use to variably adjust the location of the roving tap.
 4. The apparatus according to claim 3, wherein at least one adjustable filter coefficient, of the set of adjustable coefficients, is determined by the first transmitter in response to an adjustment received from the first receiver.
 5. The apparatus according to claim 3, wherein an adjustable coefficient, of the set of adjustable coefficients, is adjusted by a transmission, from the first receiver to the first transmitter, over a back channel that does not significantly impact the capacity to send user data in a same direction as the back channel.
 6. The apparatus according to claim 3 wherein the communications channel is a backplane.
 7. A method for transmission of data through a communication channel between a first transmitter and a first receiver, comprising: determining an adjustment in the first receiver; determining in the first transmitter, in response to the adjustment received through a back channel within an overhead associated with an encoding technique, an adjustable filter coefficient of a set of adjustable coefficients; and wherein the set of adjustable coefficients, that applies to taps located on the first transmitter, includes an adjustable filter coefficient for a movable tap location, an adjustable delay coefficient for the movable tap location and an adjustable filter coefficient for a fixed tap location.
 8. The method of claim 7, wherein the communications channel is a backplane.
 9. A method for transmission of data through a communication channel between a first transmitter and a first receiver, comprising: determining, at the first receiver, a correlation of error; adjusting a first adjustable coefficient, of a set of adjustable coefficients, to reduce the correlation of error; wherein the set of adjustable coefficients, that applies to taps located on the first transmitter, includes an adjustable filter coefficient for a movable tap location, an adjustable delay coefficient for the movable tap location and an adjustable filter coefficient for a fixed tap location, and wherein the adjustable delay coefficient is used to variably adjust the location of the roving tap.
 10. The method of claim 9, further comprising: determining an adjustment in the first receiver; and determining in the first transmitter, in response to the adjustment, the first adjustable filter coefficient.
 11. The method of claim 9, wherein the step of adjusting further comprises: transmitting, from the first receiver to the first transmitter, over a back channel that does not significantly impact the capacity to send user data in a same direction as the back channel.
 12. The method of claim 9, wherein the communications channel is a backplane. 